This invention relates to anionic surface sizing agents for paper based on maleic acid anhydride/diisobutylene copolymers which have a minimal tendency towards foaming under normal processing conditions coupled with a wide scope of application depending on the papers used. German Offenlegungsschrift Nos. 2,361,544 and 2,501,123 describe the production and use of surface sizing agents for paper having excellent properties. The sizing agents in question are aqueous or aqueous-alcoholic alkali metal, amine or ammonium salt solutions of substantially equimolar copolymers, produced in solution or suspension, of maleic acid anhydride and diisobutylene of which from 10 to 100 mole percent of the anhydride groups may be esterified with linear or branched aliphatic monohydricalcohols containing from 1 to 20 carbon atoms or with cycloaliphatic monohydricalcohols containing 5 or 6 carbon atoms to form the corresponding semi-esters. It is also known (cf. German Offenlegungsschrift No. 2,701,760) that, in the solution or suspension copolymerisation of maleic acid anhydride and diisobutylene, a monomer corresponding to the following general formula: ##STR1## wherein R represents --H, --CH.sub.3, --C.sub.2 H.sub.5, --OR.sub.2, --OCOCH.sub.3, ##STR2## --CH.sub.2 OH or --CH.sub.2 --O--COCH.sub.3 ; R.sub.1 represents --H, or --CH.sub.3 ; and
R.sub.2 represents a linear or branched alkyl group containing from 1 to 4 carbon atoms;
or mixtures of these monomers with one another may be added to the polymerisation mixture shortly before or after completion of copolymerisation in quantities of from 1 to 12 mole percent, based on the maleic acid anhydride used, excellent anionic surface sizing agents also being obtainable in this way providing the aqueous or aqueous-alcoholic alkali metal, amine or ammonium salt solutions are prepared therefrom. From 10 to 100 mole percent of the anhydride groups of these substantially equimolar copolymers may also be esterified to form the corresponding semi-esters by reaction with linear or branched aliphatic monohydricalcohols containing from 1 to 20 carbon atoms or with cycloaliphatic monohydricalcohols containing 5 or 6 carbon atoms.
Although all the surface sizing agents for paper produced by the above-mentioned process show excellent sizing properties on unsized and pre-sized papers, they are nevertheless attended by certain disadvantages in practical application, for example the foaming which these products undergo under certain practical conditions. foaming occurs above all when the sizing liquid, which in addition, to the sizing agent may also contain up to 8%, by weight, of starch, is prepared using hard water. In addition, the sizing liquid also shows an increased tendency towards foaming when the construction of the particular size press used promotes the entrainment of air bubbles into the sizing liquid. This may be caused, for example, by an excessive drop during recycling from the size press, by the use of rotary pumps instead of reciprocating pumps for transporting the sizing liquid or even by an unfavourable design of the stirrer used in the working vessel. In addition, the type of starch used in the sizing liquid also has a significant bearing upon the foaming behaviour of the above-mentioned sizing agents.
The foaming which occurs in the sizing liquid and on the size press has an adverse effect upon the paper manufacturing process. The particles of foam thrown out by the size press may cause specks in the paper and, in some cases, may even result in tearing of the paper web with the result that production might have to be interrupted.